An EU team examined the relationship between cognitive capacity and social complexity in monkeys. Although differences between species of high and low social complexity were generally minor, high complexity species had advanced social cognition.
According to Charles Darwin, and subsequent proof, humans share certain cognitive capacities with animals, while others are uniquely human. A social intelligence hypothesis may explain some of the differences, whereby cognitive capacity could relate to the complexity of various species’ social environments.
Further details: Social cognition in monkeys